Method for improving the imprintability of synthetic material



Oct. 9, 1962 H. FROHLICH 3,057,792

METHOD FOR IMPROVING THE IMPRINTABEELITY OF SYNTHETIC MATERIAL FiledDec. 22, 1958 5 Sheets-Sheet l Fig.1 1

Oct. 9, 1962 H. FROHLICH 3,05 7,792.

METHOD FOR IMPROVING THE IMPRINTABILITY OF SYNTHETIC MATERIAL Filed Dec.22, 1958 3 Sheets-Sheet 2 Fig.2 1

H. FRUHLICH Oct. 9, 1962 METHOD FOR IMPROVING THE IMPRINTABILITY OFSYNTHETIC MATERIAL Filed Dec. 22, 1958 3 Sheets-Sheet 3 3,057,792Patented Oct. 9, 1962 Free 31,857,792 METHUD FQR IMPROVENG THEEMPRINTABILETY F SYNTHETKI MATERIAL Heinz Frtthlieh, Erlangen, Germany,assignor to Siemens- Schnckertwerke Atrtiengeseiisehaft,Berlin-Siemensstadt, Germany, a corporation of Germany Filed Dec. 22,1958, Ser. No. 782,125 Claims priority, application Germany Dec. 21,1957 4 Claims. (Cl. 204-165) My invention relates to a method forimproving the imprintability of synthetic plastics by modifying theirsurface texture for better adherence of printing dyes.

Generally, the adherence of such dyes to the surfaces of syntheticmaterials is rather poor, some of these materials, such as polyethylene,being not readily imprintable unless the surface is first subjected tospecial processing. According to one of the known processes, thesynthetic material, for example in form of a foil, is subjected to ahigh voltage spray field in atmospheric pressure. Such a field can beproduced by applying a low or high frequency alternating voltage in theorder of several kv. to the electrodes of an air-gap capacitor with thesynthetic material to be processed located between the electrodes.

The improvement of the dye-holding ability thus obtainable is due to theaction of ozone ions and probably also due to the ions of certainoxygen-nitrogen compounds evolving from the electric discharge. Theduration of the processing period required depends upon the obtainableion concentration, an increase in degree of ionization in the dischargegap having the effect of more rapidly producing the desired result.

The degree of ionization for a given electrode geometry is determined bythe magnitude of the alternating voltage and the current intensity inthe spray discharge gap. Since the voltage cannot be increasedindefinitely and the current intensity is limited by the capacitance ofthe device, the known processing devices, operating with low-frequencyalternating voltage, required relatively long discharge distances forattaining practically applicable proc essing periods, or it wasnecessary to operate with highfrequency voltage thus incurring aconsiderable expenditure.

Furthermore, the known method and devices are successfully applicableonly with certain synthetic materials that respond to ions produciblewithin normal air, and the processing of large-size bodies of syntheticmaterial is extremely difficult because of the existing technologicaland economical limitations as to size and shape of the capacitorelectrodes.

In addition to such limitation in applicability, the known devices haveconsiderable disadvantages with respect to operational requirements.Since the processing of the synthetic materials is effected in air, adetrimental influence of variations in the humidity content of the airupon the desired effect is virtually inevitable, particularly in view ofthe fact that a continuous current of air must be passed through theapparatus for avoiding injurious effects of the ozone upon the health ofthe personnel. Furthermore, the concentration of neutral ozone in thespraytype electric discharge is relatively large because at atmosphericpressure the ions possess a great recombination probability. It cannotbe avoided, therefore, that the surface of the synthetic materialbecomes charged with neutral ozone gas which is inactive as regards thedesired result and has merely the effect of impairing the smell of thesynthetic material.

It is an object of my invention to improve the imprintability ofsynthetic materials by a method and means suitable for use in industriallarge-scale manufacture and free of the above-mentioned limitations anddisadvantages.

According to a feature of my invention, the high ion concentrationrequired for the processing can be produced in a considerably simplerand more effective manner by utilizing the plasma of a low-pressureelectric gas discharge. Despite the slight gas pressure, the ionconcentration in such discharges can be increased by increasing thecurrent density of the discharge up to any desired multiple incomparison with the ion concentration attainable in high-voltagedischarges at atmospheric pressure.

According to a more specific feature of my invention therefore, thesynthetic material to be processed for better dye-holding ability isplaced into a vacuum, and the surface of the material is subjected tothe plasma of an electric low-pressure gas discharge. The vacuum mayhave a pressure in the decimal order of magnitude of 1 mm. Hg. Thecomposition of the gases in the discharge may be controlled or keptconstant by supplying the vacuum or negative-pressure chamber with givendosages of desired kinds of gas.

A device for performing the method according to the invention mayoperate, for example, with a low-voltage glow discharge of approximately400 to 500 volts between the electrodes, or the plasma may also beproduced by thermionic cathodes operating with an arc voltage ofapproximately volts. The high current densities of such dischargespermit reducing the processing time down to fractions of thosepreviously required. For example, when processing polyethylene in anoxygen atmosphere of about 1 mm. Hg pressure a processing period of onlyOne second at a current density of 2 ma./cm. is sufiicient. With highercurrent densities, a further reduction in processing time is obtainable.The power consumption in the example just given amounts to approximatelyone-fifth of the consumption of a high-voltage spray discharge device Hof the same through-put per time unit operating at utilityline frequency(50 or 60 c.p.s.).

Since the processing takes place in vacuum, the charging of the surfacewith neutral ozone is eliminated, thus also avoiding bad smell of theproduct.

The production of the plasma may be effected with direct current oralternating current. It has been found that a high-frequency alternatingfield of 1 to 1000 megacycles per second is particularly favorable.Producing such a field involves a relatively small expenditure inmaterial and space because of the low arc voltages required. As a rule,the synthetic material is available in form of foils, sheets, tapes orother webs and hence must be passed continuously through thenegative-pressure chamber with the aid of vacuum locks. This can readilybe done with synthetic foils of any thickness or width. Applicable forthis purpose, for example, is a vacuum lock having at least onepro-vacuum and one post-vacuum chamber provided with slots for thepassage of the material, the gaps remaining between the insertedsynthetic material and the edges of the slots being smaller than themean free paths of the gas molecules in the spaces that communicate witheach other through the slots. When using such slot-type vacuum locks anysealing means can be dispensed with.

The method according to the invention is also suitable for theprocessing of bodies of synthetic materials having any desired shape,because the spacial geometry of the discharge can be chosen at will bygiving the electrodes a corresponding shape.

The foregoing and other objects and features of my invention, the novelfeatures being set forth with particularity in the claims annexedhereto, will be apparent from, and will be further explained in, thefollowing with reference to the drawings in which:

FIG. 1 illustrates schematically in longitudinal section a processingdevice according to the invention in conjunction with its electricalaccessories.

FIG. 2 shows in a similar manner a modified form of processingequipment.

FIG. 3 shows a longitudinal section through another embodiment of adevice according to the invention suitable for the processing ofthree-dimensional bodies, such as bottles or tubes; and

FIG. 4 is a cross-section along the line IV--IV indicated in FIG. 3.

The apparatus illustrated in FIG. 1 operates with a selfsustaininglow-pressure glow discharge for the processing of synthetic foils inform of a sheet or tape. The foil material 1 passes from the rightthrough a narrow entrance slot 2 whose cross-section and axial lengthare so chosen that the flow resistance in the slot suffices forproducing in the pre-vacuum chamber 3 of the device a desired negativepressure by means of pumps (not illustrated) to be connected to nipples4 located on both sides respectively of the foil 1. If desired, two ormore such pre vacuum chambers may be provided one behind the other.

Another slot 2a, designed in the same manner as the slot 2, connects thechamber 3 with the processing chamber 5 proper, the foil passinghorizontally through the center of the chamber. Located on both sides ofthe foil 1 in chamber 5 are two pairs 6 and '7 of electrodes. The twoelectrodes of each pair are connected to respective leads 8 and 9 whichpass through air-tight seals in the chamber wall to the outside wherethey are connected, in series with respective adjustable resistors it)and 11, with the secondary winding of a transformer 22 whose primarywinding is energized from an alternating current source of any desiredfrequency, such as about 50 or 60 cycles per second. A double-poleswitch 13 permits disconnecting the electrode pair 7 in cases where onlyone side of the foil is to be treated. The processing chamber 5 is connected With vacuum pumps (not shown) that communicate with the upper andlower portion of the chamber through respective nipples 14. By means ofthese pumps, the chamber 5 is evacuated down to the vacuum, preferably0.5 to 1 mm. Hg, required for producing a glow discharge between theelectrodes of each pair. The voltage required for such glow dischargedepends upon the negative pressure in chamber 5 and may amount to a fewhundred volts, for example.

The device is preferably provided with gas supply nipples 15 located onboth sides of the foil 1 and on both sides respectively of theprocessing chamber 5. The nipples 15 permit supplying the device withthe gas or gas mixture that is to be active in the processing chamber 5.A supply of oxygen is suitable for the processing of polyethylene, butnon-oxidizing gases are also applicable, chlorine gas being suitable formost synthetic materials other than polyethylene. The nipples 15 areclosed when no such additional gas is needed. The chamber 5 is connectedthrough another slot 21;, corresponding to those described above, with apost-vacuum chamber 16. The chamber is preferably provided with nipples17, which permit adjusting the negative pressure in the chamber to anydesired value by means of a pump.

By maintaining in the gas supply nipples 5 a gas pressure higher thanthe air pressure existing at these respective locations of slots 2a and2b, the ingress of atmospheric air into the processing chamber 5 can besubstantially or fully prevented if this is required for best processingresults. In this case, and in the absence of oxygen, it is also possibleto use, instead of the alternatingcurrent energized, self-sustaininglow-pressure glow discharge, a low-pressure arc discharge as isillustrated in the modified device shown in FIG. 2.

The device of FIG. 2 is to a large extent similar to that of FIG. 1, thesame reference numerals being used in both illustrations for similarcomponents respectively. However, the device shown in FIG. 2 is providedwith two thermionic cathodes 13 and cooperating with respective anodes19 and 21. The cathodes IS and 26 are connected by leads 22 and 23 witha transformer 25 for heating the two cathodes from an alternatingvoltage source. The leads 22 and 23 pass through gas-tight seals in thewalls of the processing chamber 5. The anodes It? and 21 are connectedby respective leads 3 and 9 through adjustable resistors it} and illwith the positive pole of a direct-current generator 24 whose negativepole is connected to the cathodes 16 and 26. A switch 13 permitsdisconnecting the lower arc gap if only one side of the foil l i to betreated. The voltage between each cathode and anode may amount to a fewhundred volts depending upon the gas pressure in the processing chamber5.

The device illustrated in FIGS. 3 and 4 permits the processing of suchbodies of synthetic material as bottles, collapsible tubes and the like.The device comprises a housing structure 31 which forms a vacuum chamberin its interior and can be closed and vacuum-tightly sealed by means ofa front door 32. Located between the side walls of housing 31 are twoinsulating plates 34 which are provided with parallel longitudinalgrooves 40 for the insertion of metallic electrodes 33, preferablyconsisting of planar metal plates. The electrodes 33 engage respectivecontacts 36 mounted on vertical connector strips 35. The contacts 36supply the electrodes 33 with electric voltage whose polarity changesfrom plate to plate. The voltage is applied to the contacts 36 throughleads 41 passing through vacuum-tight seals in the rear wall of thehousing structure 311. During operation a glow discharge is maintainedbetween the electrodes 33.

The electrodes 33 are preferably designed as perforated plates forfacilitating the evacuation of the processing chamber. However, theelectrodes 33 may also consist of Wire mesh, grid-shaped gratings, or ofnon-perforate plates or sheets which need not necessarily be planar, butmay be given any desired shape corresponding to the shape of thematerial or bodies to be processed. If necessary, the electrode platesmay carry inserts of any desired shape (not illustrated) correspondingto the shape of the bodies to be processed, the inserts being inelectrically conducting connection with the electrodes.

The spacing between the electrodes 33 can be adapted to the size of thebodies of synthetic material to be processed, by inserting the electrodeplates in properly chosen grooves 4%) of the insulating plates 34. Thebodies to be processed are placed upon the electrode plates, or are heldby means of inserts or parts placed upon the electrode plates orsuspended therefrom. The vacuum pump (not illustrated) for evacuatingthe processing chamber in housing 31, can be connected to a nipple 37.The device is further provided with a valve 38 for admitting atmosphericair, and with a pressure measuring instrument 39.

If desired, devices according to the invention can readily be automatedby providing them with electric and vacuumresponsive sensing means forautomatically controlling the pumping operations, the starting of theglow or are discharge in dependence upon the vacuum pressure, and thesubsequent ventilating of the processing chamber. It will also beapparent to those skilled in the art, upon a study of this disclosure,that devices according to my invention may be modified in various otherrespects and hence may be given embodiments other than thoseparticularly illustrated and described herein, Without departing fromthe essence of the invention and Within the scope of the claims annexedhereto.

I claim:

1. The method of improving the dye-holding ability of syntheticplastics, which comprises placing the synthetic material in a vacuum andsubjecting its surface to the plasma of an electric low-pressure glowdischarge of a few hundred volts.

2. The method of improving the dye-holding ability of syntheticplastics, which comprises placing the synthetic material into a vacutunchamber having a pressure in the decimal order of magnitude of 1 mm. Hg,supplying a non-oxidizing gas to the chamber while maintaining saidpressure, and subjecting the surface of the material to the plasma of anelectric low-pressure glow discharge of a few hundred volts.

3. The method of improving the dye-holding ability of syntheticplastics, which comprises placing the synthetic material into a gaseousatmosphere having a pressure in the decimal order of magnitude of 1 mm.Hg, and subjecting the surface of the material to the plasma of anelectric self-supporting glow discharge of a few hundred volts.

4. The method of improving the dye-holding ability of syntheticplastics, Which comprises placing the synthetic material into a gaseousatmosphere having a pressure in the decimal order of magnitude of 1 mm.Hg, and subjecting the surface of the material to the plasma of athermionic arc discharge of approximately one hundred volts.

References Cited in the file of this patent UNITED STATES PATENTS1,888,419 Ainsworth Nov. 22, 1932 2,454,757 Smith Nov. 23, 19482,468,173 Cotton Apr. 26, 1949 2,551,035 Miller May 1, 1951 2,837,654Berghaus et al. June 3, 1958 2,864,755 Rothacker Dec. 16, 1958 2,881,470Berthold et al Apr. 14, 1959

1. THE METHOD OF IMPROVING THE DYE-HOLDING ABILITY OF SYNTHETICPLASTICS, WHICH COMPRISES PLACING THE SYNTHETIC MATERIAL IN A VACUUM ANDSUBJECTING ITS RANGE TO THE PLASMA OF AN ELECTRIC LOW-PRESSURE GLOWDISCHARGE OF A FEW HUNDRED VOLTS.